1. Field of the Invention
This invention relates to an optical type measuring device, and more particularly to improvements in an optical type measuring device, wherein a parallel scanning light beam is utilized to measure dimensions of a workpiece to be measured.
2. Description of the Prior Art
Heretofore, there has been adopted an optical type measuring device wherein a rotary scanning light beam (a laser beam) is converted by a collimator lens into a parallel scanning light beam to be passed through this collimator lens and a condensing lens, a workpiece to be measured is interposed between the collimator lens and the condensing lens, and dimensions of the workpiece to be measured are measured from the time length of a dark portion or a bright portion generated due to the obstruction of the parallel scanning light beam by the workpiece to be measured.
More specifically, as shown in FIG. 1, a laser beam 12 is oscillated from a laser tube 10 toward a stationary mirror 14, the laser beam 12 thus reflected is converted into a rotary scanning light beam 17 by a polygonal rotary mirror 16, the scanning beam 17 is converted into a parallel scanning light beam 20 by a collimator lens 18, a workpiece 24 to be measured interposed between the collimator lens 18 and a condensing lens 22 is scanned at high speed by the parallel scanning light beam 20, and dimensions in the scanning direction (direction Y) of the workpiece 24 to be measured are measured from the time length of a dark portion or a bright portion generated due to the obstruction of the parallel scanning light beam by the workpiece 24 to be measured.
More specifically, the bright and dark portions of the parallel scanning light beam 20 is detected as variations in output voltage of a light receiving element 26 disposed at the focal point of the condensing lens 22. Signals from the light receiving element 26 are fed to a pre-amplifier 28, where they are amplified, and then, fed to a segment selector circuit 30. This segment selector circuit 30 is adapted to generate a voltage V to open a gate circuit 32 only for a time t, during which the workpiece 24 to be measured is scanned, from the output voltage of the light receiving element 26 and feeds the same to the gate circuit 32. A continuous clock pulse CP is fed to this gate circuit 32 from a clock pulse oscillator 34, whereby the gate circuit 32 generates clock pulses P for counting the time t corresponding to the dimensions in the scanning direction, for example, the outer diameter of the workpiece 24 to be measured and feeds the same to a counter circuit 36. Upon counting the clock pulses P, the counter circuit 36 feeds a count signal to a digital indicator 38, where the dimensions in the scanning direciton, i.e., the outer diameter of the workpiece 24 to be measured is digitally indicated.
On the other hand, a synchronous motor 44 is synchronously driven by an output of a synchronous sine wave oscillator 40, which generates sine waves in synchronism with the clock pulse oscillator 34 and a power amplifier 42. So, the synchronous motor 44 rotates the polygonal rotary mirror 16 in synchronism with the continuous clock pulses CP fed from the clock pulse oscillator 34, whereby the measuring accuracy is maintained.
The above-described high speed scanning type laser length measuring device has been widely utilized because the lengths, thickness and the like of moving workpieces and workpieces heated to a high temperature can be measured at high accuracies in non-contact relationship therewith.
However, in the polygonal rotary mirror 16 in the above-described high speed scanning type laser length measuring device, a distance in the scanning direction from the reflecting point 16A to the optical axis 18A of the collimator lens 18 varies periodically during its rotation as enlargedly shown in FIG. 3, such a disadvantage is presented that the measuring accuracy varies.
In contrast thereto, the rotary mirror 16 is formed into a single plane mirror, so that the distance from the reflecting point can be prevented from varying. However, the above-described single plane mirror is limited in the number of cycles of scanning the work to be measured by the laser beam, so that the averaged accuracy of the measured values cannot be improved.
In consequence, in the above-described measuring device, it becomes inevitable to utilize the polygonal rotary mirror.
As shown in FIG. 4, when an angle made by a rotary scanning light beam 17 and the optical axis of the lens is .theta., the collimator lens 18 is normally made to be a so-called f lens wherein the light beam exiting from the collimator lens 18 is changed into a parallel scanning light beam 20 having a distance y=f.theta. (f is a focal length of the lens) from the optical axis of the lens. In order for the exiting light beam to become parallel to the optical axis of the collimator lens 18 as described above, it is on the assumption that the reflecting point of the laser beam 12 on the polygonal rotary mirror 16 is positioned at a focus F.sub.1 on the inlet side of the collimator lens 18.
However, since a distance from the rotary center to the reflecting surface of the polygonal rotary mirror 16 varies periodically as described above, the light beam of the collimator lens 18 on the outlet side is periodically shifted in angle relative to the optical axis of the lens. For this reason, the measuring accuracy varies.
For example, in the case where the diameter .phi. of a collimator lens 18 is 30 millimeter, the focal length f is 90 millimeter and a normal distance R from the rotary center to the reflecting surface of the polygonal rotary mirror 16 is 9 millimeter, when a shift of the reflecting point on the incident optical axis is .DELTA. and an inclination angle of an exiting beam to the optical axis of the lens is .alpha., and if errors in measurement are 0.5 micrometer and 1 micrometer, then a measurable range is shown in the following Table 1.
As apparent from this table 1, if a measuring range is determined under a predetermined measuring errors tolerance, then a scanning angle .theta. is determined. The size of the work to be measured is determined by an effective diameter of the collimator lens 18, whereby y=f.theta. becomes smaller in value than the effective diameter of the lens. In consequence, over a predetermined measuring range, if the scanning angle .theta. becomes small, then the focal length f of the collimator lens 18 should be increased. An increased focal length f of the collimator lens 18 disadvantageously result in an increased size of the measuring device.
TABLE 1 ______________________________________ Scan- Shift of Measuring ning Beam reflecting Inclination range angle height point of beam 0.5 .mu.m 1 .mu.m .theta. .gamma. .DELTA. .alpha. Tolerance Tolerance ______________________________________ 2.degree. 3.141.sub.6 1.37 .mu.m 0.11 sec. 937.6 1875.1 4.degree. 6.283.sub.2 5.49 0.88 117.4 234.8 6.degree. 9.424.sub.8 12.35 2.96 34.8 69.7 8.degree. 12.566.sub.4 21.98 7.03 14.7 29.3 9.5.degree. 14.922.sub.6 31.02 11.79 8.7 17.5 ______________________________________